The present invention relates to technology of forming a fine light beam, such as technology that can be effectively adapted for enhancing the data recording density of an optical disk or an optomagnetic disk, enhancing the resolution of an optical microscope apparatus, enhancing the precision of measurement at the time of measuring the height and size of the surface of a sample, enhancing the precision of inspection at the time of inspecting the presence of foreign matter or defects on the surface of the sample, enhancing the precision of transferring fine patterns in an electron beam drawing device or an optical exposure device, and for enhancing the precision of machining at the time of finely machining the surfaces of semiconductor wafers or semiconductor chips.
In an optical apparatus such as an optical disk apparatus or an optical microscope apparatus, a system has in many cases been employed in which light emitted from a source of light is transformed into a parallel beam through a collimator lens and is focused by an objective lens in order to form a light beam.
In this case, if it is attempted to make very small the spot diameter of the light beam focused by the objective lens, there is a limitation due to the diffraction phenomenon caused by the wavelength of light and the numerical aperture of the objective lens as is widely known.
The diameter (Wo) of a light beam that can be focused by an objective lens is determined according to the following equation: EQU 2Wo=k.lambda./NA
where
NA is the numerical aperture of the objective lens, PA1 and .lambda. is the wavelength of light, as described in, for example, "Optical Fiber Application Technology", Nikkei Gijutsu Tosho Co., Dec. 10, 1986, pp. 431-435. Furthermore, the symbol k is a constant that is determined by the superposition condition of the diameter of the objective lens and the distribution of intensity of the light beam, and has the following value: PA1 k=0.5 (when the half-width of the light beam is equal to the diameter of the lens), or PA1 k=0.8 (when the e.sup.-2 intensity width of the light beam is equal to the diameter of the lens).
As described above, the lower limit of the spot diameter of the light beam focused by the objective lens is determined depending upon the wavelength of the light, characteristics of the objective lens, and superposition of the intensity distribution of the light beam.
It is therefore impossible to form a light beam having a diameter smaller than the above limit value, placing a limitation on enhancing the recording density of the optical disk apparatus and on enhancing the resolution of the optical microscope apparatus.
Technology that utilizes a fine light beam includes exposure technology of transferring a pattern of an integrated circuit onto a semiconductor wafer and technology of measuring the height and size of the surface of a sample or of inspecting the appearance of the surface of the sample in addition to the aforementioned optical disk device and the optical microscope apparatus.
In the step (wafer process) of manufacturing a semiconductor integrated circuit device, for example, a pattern of an integrated circuit formed on a photomask (reticle) is transferred onto a photoresist on a semiconductor wafer to form a resist pattern, and a thin film is removed by etching using the resist pattern as a mask or impurity ions are implanted into the semiconductor substrate, in order to form a desired LSI.
In this case, the dimensional precision of the pattern of the integrated circuit formed on the photomask is a factor that greatly affects the yield and reliability of LSIs.
Technology of measuring the size by using a laser beam has heretofore been put into practical use in the step of inspecting the pattern of the integrated circuit formed on the photomask. This is a method of measuring the size of a pattern by irradiating the pattern on the photomask with a laser beam to effect the scanning in the direction of measurement of the size and by detecting a peak position of scattered light or a position at which the reflection factor changes.
In this case, for example, the laser beam is slightly vibrated in the direction of measurement of the size, and the position of scattered light or reflected light is detected in synchronism with this slight vibration in order to improve the precision of measurement.
In the above-mentioned technology of measuring the size using a laser beam, however, there is a problem that it is no more possible to sufficiently focus the beam due to the limitation caused by the wavelength of the light and the numerical aperture of the objective lens when the size of the integrated circuit pattern formed on the photomask becomes nearly as small as the wavelength of the light although the laser beam is focused on the photomask using the objective lens. Therefore, the peak position of the scattered light and the position where the reflection factor changes are broadened, deteriorating the precision of measuring the size.
In the step of inspecting the integrated circuit pattern formed on the semiconductor wafer, furthermore, height-measuring technology has been put into practical use according to which the surface of the wafer is irradiated with a light beam obliquely, and a relative position of the reflected light is measured using an optical sensor such as a CCD (charge-coupled device).
In this case, the light from, for example, a light-emitting diode is focused and is permitted to fall on the surface of the wafer and the reflected light is permitted to be incident on the light detector via an oscillation mirror, and a detection signal obtained by the light detector is detected in synchronism using a modulation signal for driving the oscillation mirror as a reference signal, in an attempt to enhance the precision of measurement.
Even in the above height-measuring technology, however, there is a problem that it becomes no more possible to sufficiently focus the diameter of the light beam as the size of the integrated circuit pattern becomes fine due to the aforementioned limitation caused by the wavelength of the light and the numerical aperture of the object lens. Namely, the spot becomes blurred although the light reflected by the surface of the wafer is focused on the sensor, and the precision of measuring the height is deteriorated.
As a method of inspecting defects and foreign matter on the surfaces of semiconductor wafers and photomasks, furthermore, there has been known appearance inspection technology such that the surfaces thereof are irradiated with a laser beam to detect the light scattered or reflected by defects or foreign matter.
This is the technology in which the surface of a sample (wafer or photomask) is irradiated with a laser beam from a horizontal direction and the light scattered by foreign matter (or a defect) is detected by a detector such as a photomultiplier disposed above the sample.
In this case, a polarizing plate is used to separate the laser beam into two polarization beams whose polarized planes are perpendicular to each other, and the light scattered by the edges of the true pattern formed on the sample and the light scattered by foreign matter (or a defect) are separated from each other, in order to improve the precision of detection.
However, even this appearance inspection technology becomes no more capable of sufficiently focusing the beam diameter due to the limitation caused by the wavelength of the light and the numerical aperture of the objective lens as the size of the integrated circuit pattern formed on the semiconductor wafer or the photomask becomes fine and the size of the defect or foreign matter to be detected becomes fine correspondingly. Therefore, a peak position of the scattered light or a position at which the reflection factor changes is broadened, deteriorating the precision for detecting defects and foreign matter.
There has further been known defect remedy technology which corrects the wiring pattern on the semiconductor wafer using a fine laser beam. Even in this case, however, a limitation is imposed on decreasing the size of the laser beam due to the aforementioned reasons, and the precision of machining the wiring is deteriorated as the size of the wiring pattern becomes fine.